BRAKES LOCKING UP

[spitfun]

I ran my car at the first autocross of the year today. After the first run the front brakes started to lock up to the point where they were smokingi!! I did bleed the brakes over the winter and may have put in DOT4 or DOT5,is this possibly the cause? The cap says DOT3 by the way. I had to wait for the brakes to cool before it could be driven into the trailer-locked up solid,both of them.The rears seem fine. Any ideas guys?? Thanks Rich

[rad]

Hi. Simple thing, shorten the brake master cylinder pushrod so you have 1/8 inch free play. There is more but this is most common.

[spitfun]

So you think the sutup is too tight? This car has been around for ten years with another owner-it seems like this problem would have surfaced a long time ago...

[carguy123]

Sounds like rad is right.

If it's too tight then it gradually "tightens" the brakes down as it doesn't fully release.

[spitfun]

Thanks for the tip. I will investigate that option. Rich

[rad]

Hi. Little more detail.
When the pushrod is too tight, the brake fluid when it gets heated up cannot return to the fluid reservoir.

[john hennessy]

how old is the fluid, its hydroscopic and absorbs moisture, you may have reached a point where so much moisture is absorbed that it boils and holds the brakes on.

when it starts to lock up, look in the master cylinder and see if you can see turbulence in the fluid, if yes then the fluid is returning, if no then the push rod is too long or the brake switch is holding the pedal down, if there are bubles then the fluid is probably contaminated with too much moisture.

[BBlue]

It could be the piston is simply sticking and trapping the fluid.

Bill

[cheapracer]

Sounds like a simple bit of corrosion in the master cylinder bore or caliper bores (although 2 at the same time is unusual).

Time to run the hone down the master cylinder and throw a new kit through it by the sound of it, should take you about 3 or 4 beers. Do the calipers too while you're at it.

[romkasponka]

If all the wheels are breaking when they should not do it it is definitely pushrod adjustment problem. I had the same problem on my car. When the fluid becomes hot it expands and locks your brakes.

[a.moore]

If it was fine before, I don't see how the push-rod will need adjusted unless something bent or the jam nut came loose and it self adjusted (been there, done that).

I'll second CR's suggestion to check the master cylinder bore for corrosion.

[raceral]

I agree especially if the pads have worn down allowing the piston to extend further into the cylinder than normally.

Al

[vroom]

I'm with John. Unless of course you mixed DOT 3 or 4 with DOT 5 (silicone based) which are not compatible. Silicone is also hard to bleed, a possible.

[john hennessy]

are you using a single piston caliper or dual pistons?

if single piston, is the caliper floating correctly?

do you have free play at the pedal?

is your brake light switch closed when the brakes lock up, are the lights on?

is the brake light switch mechanical or pressure activated?

what master cylinder/cylinders are you using?

try this, apply the brakes, hard, release the pedal, now open the bleed nipple, does fluid squirt out and the brake release?

if the above happens, close the bleed nipple, apply the brakes again, hard, now open the pipe at the master cylinder, does fluid squirt out and the brakes release?

if the first test proves true then the calipers are probably good, if the brake does not release then the calipers are probably siezed or not floating.

if the second test proves true, then the flex hoses are probably good and your problem is at the master cylinder, if not, then the flex hoses may be collapsing inside or a metal pipe has been crushed or is blocked by dirt.

now with the pedal released and the brake line disconnected from the master cylinder, apply a small amount of air pressure to the port, breath pressure should be enough, can you see bubbles in the resovoir, if the push rod is too long, or the piston is siezed in the bore, no air will be passed to the resovoir.

remove the master cylinder bolts from the mounting bracket and pull the master cylinder out as far as you can, do the breath test again, if bubbles now appear in the resovoir then your push rod or pedal adjustment is at fault, if no bubbles appear then the master cylinder is either blocked, the piston is stuck or the rubber seals are damaged.

if all the above tests prove good, then i believe that the problem is down to the disc/caliper relationship, especially if it only occures when hot because the piston is retracting into the caliper but the alignment is incorrect and the pad/piston/disc/caliper is a different size when hot and no longer centralized.

alternatively but unlikely, is the brake pedal heavy, how much does it weigh, is its own weight causing it to bounce and pump up the brakes?

[spitfun]

The calipers are Wilwood Dynapro's dual pistons. I think you guys are on to something about the freeplay. I doubted this but went out to the car and depressed the clutch and the brake pedals. The clutch has a little play where the brakes feel like the pedal is right on the pushrod. I am going to space out the master a little with some washers and bleed out all the old fluid-there are two reservoirs so it shoulbd be pretty easy,might as well do the rear to just to get some new fluid all around. The car has been built for 10 years so I was doubtful about the freeplay,maybe there is too much water in the old fluid and it is boiling and causing the pushrod to need more freeplay to take up the extra space in the lines? I will keep you informed on how it goes... Rich